The present invention relates to a voice coding communication system and apparatus therefor and, more particularly, to a voice coding communication system and apparatus therefor which apply an adaptive differential pulse code modulation (hereinafter referred to simply as ADPCM) system to their coding system.
It is said that the voice activity factor of either the calling or called party in voice communication is 35% or so.
In recent years, personal communication, mainly from person to person, has become widespread. The personal communication is usually voice communication which utilizes portable terminal equipments. A first requirement for such portable terminal equipments is to make them cordless and a second requirement is to reduce power consumption of their circuits so as to lengthen the service life of batteries which are employed with a view to making the terminal equipments easy to carry.
One possible method that has been proposed to reduce the power consumption more than in the past is to actuate the transmitter only during voice activity and hold it out of operation during the other transmission time, taking the voice activity factor into account. This method could be implemented by equipping the transmitting side with a voice activity detecting function and a discontinuous transmitter. This method, however, poses a problem at the receiving side. That is, the reproduced voice at the receiving side is discontinuous and hence is very uncomfortable. As regards the cause of this problem, it is well-known that during transmission of voices they are superimposed on background noise, whereas when no voice is being transmitted the background noise is not transmitted either and that only during the duration of a voice signal the background noise is modulated and transmitted together with the voice signal.
As a solution to this problem, it is well-known to generate a pseudo-noise similar to the background noise at the transmitting side while no voice signal is transmitted to the receiving side. Such a technique was studied first in the field of digital communication employing a low bit rate coding method based on a speech analysis-synthesis technique which analyzes and transmits a voice signal and synthesizes the voice signal at the receiving side, and this technique has now become well-known in some limited technical fields, with a standardization algorithm established.
The speech analysis-synthesis typed low bit rate coding method is one that transmits the input voice signal as a short-time (about 20 msec) spectrum envelope parameter and parameters such as a distinction between voiced and unvoiced sounds and a pitch period. This method is effective in the generation of a pseudo-noise similar to a relatively gently changing background noise and will be implemented in digital automobile telephone.
On the other hand, an object of application of the present invention is a voice coding communication system of the ADPCM system which is one of speech waveform coding techniques now used as standard methods for personal communication, and a method which generates, at the receiving side, a pseudo-noise similar to the background noise at the transmitting side as mentioned above has not as yet been applied to the voice coding communication system.
The ADPCM coding system is now used in a wire transmission line (or public telecommunication network) as a substitute for the conventional PCM system. The reason for this is that the PCM system has a transmission rate of 64-kbit/s, whereas the ADPCM system has a transmission rate of 32 kbit/s but provides the same speech quality as is obtainable with the PCM system and hence permits a two-fold increase in the utilization efficiency of the circuit used.
The reason for the use of the ADPCM system in personal communication is that present-day personal communication is ranked as an extension of the public telecommunication network, such as the existing cordless telephone for domestic use, and is regarded as part of the public telecommunication network unlike the digital automobile telephone. Therefore, the techniques of detecting the voice activity during coding and generating the pseudo-background noise at the receiving side during decoding as mentioned above have not been studied with respect to the ADPCM system. This is because it has been applied to the wire transmission and hence have not been restricted by its power supply.